In recent years, risk assessment using the technique of risk-based inspection (RBI) that takes risks into consideration is performed in large facilities such as petrochemical plants and thermal power plants (it should be noted that an assessment technique based on RBI is standardized as API581 by the American Petroleum Institute (API)). As a risk assessment means of this type, JP 5884000B (Patent Document 1) discloses creating a plot diagram by plotting, with respect to each process device that is provided in a steam plant, a pair of the probability of malfunction and the degree of influence of malfunction, on a risk matrix in which the probability of malfunction and the degree of influence are set as two axes, and performing risk assessment of the target plant using the plot diagram.